‘Vsain tal umatar’ [give dew and rain] which is said in the 7^th bracha of Shmone Esrei we start saying on the third day of Cheshvan. R’ Gamlial says the 7^th of Cheshvan. This was 15 days after Succos so that the people who came to Yerushalayim would have had time to get back to the Euphrates River.

Taanis 1:4

If by the 17^th of Cheshvan the rains did not come, individuals started to fast a total of 3 fasts. They started fasting in the morning and were permitted to do work, take baths, smear oils, wear shoes and have marital relations.